The present invention relates generally to an atmospheric tritium sampler and more particularly to an atmospheric tritium sampler which utilizes a catalyst to combust tritium in gaseous form.
Work toward the goal of fusion energy production is progressing in a number of laboratories throughout the world. In the area of magnetic confinement fusion, the major effort is focused on the tokamak device, which consists of a toroidal vessel that contains a high temperature plasma, surrounded by magnetic field coils that confine and position the plasma.
The three largest tokamaks are the JT-60 in Japan, the Joint European Torus (JET) in England, and the Tokomak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory in the United States. Experiments have been conducted on all of these machines using plasmas of hydrogen and deuterium. The TFTR and JET machines plan to use a deuterium tritium mixture as a plasma fuel, which will greatly increase neutron production. Next generation devices, such as the Compact Ignition Tokomak (CIT) device which is proposed to be built at the Princeton Plasma Physics Laboratory, will also use a deuterium-tritium plasma as fuel.
The use of tritium in these machines necessitates the need for environmental tritium monitoring. Therefore, the need arises for a continuous duty air monitoring system which is capable of collecting tritium in vaporous (HTO) and gaseous (HT) forms and is suitable for environmental purposes.
An atmospheric tritium monitoring system may also be useful for sampling the air around fission reactors, particularly breeder reactors which produce tritium.
Various schemes are available for tritium monitoring. U.S. Pat. No. 4,244,783, issued to James A. Corbett, discloses a tritium monitoring system capable of monitoring the tritium content in a reactor fluid. The arrangement taught by Corbett monitors the tritium content by measuring an electric current generated by the tritium in an ionizable medium. U.S. Pat. No. 4,683,749, issued to Norman C. Thurlow, discloses a process and system for detecting and measuring a tritium gas leak from a container.
A system for measuring the tritium content in ambient air is disclosed in U.S. Pat. No. 4,638,674, issued to Eckhard Redmann. The scheme disclosed by Redmann measures the tritium content via the T.sub.2 O vapor content in the air sample. This system however neglects the gaseous tritium (HT fraction).
Various monitors exist which are capable of sampling air for HT. Generally these systems consist of a small vacuum pump which samples the air at a known flow rate. The sample air is drawn through a drying column where its moisture is removed and then passes through a heated catalyst, where any gaseous tritium which may be present is converted into water. This "manufactured water" is picked up by the sample air which subsequently passes to another moisture trap where it is adsorbed.
Several drawbacks exist with this arrangement. The first moisture trap, or HTO trap, may not collect all of the water vapor; therefore, some of the vapor in the air may be transferred to the second, or HT trap. This will result in an underestimate of the HTO fraction and an overestimate of the HT fraction. Typically, these tritium monitoring systems utilize an external heat source to heat the catalyst. The use of this external heat source is a further disadvantage to these systems. The external energy source, typically provided by an AC power source, renders these systems impractical for continuous duty in the field, where AC power may not be available.
Another major problem with these systems is the minute quantities of water manufactured by the catalyst from the gaseous tritium in the air sample. At a HT concentration of 1 maximum permissible concentration (MPC) the total manufactured water would amount to less than one microgram after a full week's run. Thus, any loss of the manufactured water before reaching the second moisture trap would result in a major discrepancy.
Systems such as the one disclosed by H. G. Ostlund et al. in "Atmospheric HT and HTO", Tellus XXVI (1974), solve this problem by feeding hydrogen gas to a catalyst. The catalyst combusts the hydrogen along with any tritium gas present in the air sample. The use of the externally supplied hydrogen results in gram quantities of manufactured water, which efficiently transport the microgram quantities of manufactured tritiated water to the drying column.
These systems require a very low hydrogen flow rate. Low hydrogen flow rates are difficult to measure with inexpensive rotometers. Therefore, these systems, which feed hydrogen gas to the catalyst, use an electrolysis cell to supply the hydrogen gas. The rate of hydrogen production is controlled by carefully keeping voltage and current supplied to the electrolysis cell at a constant level.
These systems also have several disadvantages. First, they require an external source to supply the current needed for the electrolysis process. This requirement makes these systems less than ideal for use in a continuous mode. Second, provisions have to be made to assure that the hydrogen concentration level is kept below 4%. Otherwise, the sampler will explode when the gas/air mixture reaches the catalyst. Third, the cell needs to be refrigerated during warm weather, if sampling times are long.
There are further disadvantages in the particular system disclosed by Ostlund et al. In this particular arrangement the hydrogen is catalytically oxidized in a combustion trap. The combustion trap, which is comprised of a bed of palladiam carried on a molecular sieve, serves dual functions. The combustion trap, both oxidizes the hydrogen/tritium into water and acts as a moisture trap. For sample recovery, the molecular sieve needs to be heated, under vacuum, to over 500.degree. C. for up to eight hours. Further, since no sample recovery is ever complete, a "tritium memory" may present a problem with the accuracy of future sampling runs. Therefore, in view of the above, it is an object of the present invention to provide an atmospheric tritium sampler capable of being continuously operated in the field.
It is a further object of the invention to provide an atmospheric tritium sampler capable of detecting the fraction of tritium in gaseous form from an air sample.
It is another object of the invention to provide an atmospheric tritium sampler which does not use an external heat source or an external AC power source.
It is still another object of the present invention to provide an atmospheric tritium sampler which avoids memory effects.
It is still a further object of the present invention to provide an atmospheric tritium sampler which may be safely operated and does not use explosive components.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities in combinations particularly pointed out in the appended claims.